The present invention relates generally to systems and methods for the quantitative determination of oxygen in liquid fuels, and more particularly to system and method for quantitation of dissolved molecular oxygen (O.sub.2) in fuel by observing the quenching by dissolved oxygen of the fluorescence of a laser excited probe molecule in the fuel.
Prior art methods for determining dissolved oxygen concentration in liquid fuel are cumbersome and time-consuming as to be rendered substantially useless for many measurements. For example, methods based on gas chromatography (GC) and combined GC and mass spectrometry (MS) are sensitive to at least a few parts per million (ppm) O.sub.2 and give results with fairly high precision. However, in the GC based method, the oxygen must be separated from the fuel prior to introduction to the GC column and, because any fuel in the GC sample degrades column efficiency, the column must be run through a heating cycle regularly to remove small amountsof fuel; GC is relatively slow and does not allow study of rapidly time-varying signals and is performed off-line which prevents in-situ and spatially resolved sample measurements; and GC based methods, and GC/MS based methods in particular, are relatively expensive to perform.
Electrochemical methods, such as potentiometry and voltammetry, can be used for analysis of oxygen. Oxygen is a reducible species detectable with high sensitivity by polarography in aviation fuel because of a paucity of other reducible species in the fuel; this method is relatively inexpensive and requires no separation of the oxygen from the fuel prior to measurement, but the interface between the fuel and the electrochemical cell is cumbersome and the measurement is slow, which substantially prevents use of the method for rapidly time-varying signals, and cannot be performed non-invasively. Oxygen is difficult to measure spectroscopically in organic solutions because O.sub.2 does not absorb in the infrared and has electronic transitions in the far ultraviolet where organic solutions absorb strongly. Although O.sub.2 has a Raman allowed transition and unique electron spin resonance, methods based on these attributes have low sensitivity, high cost and experimental complexity.
The invention solves or substantially reduces in critical importance problems with prior art methods as just described by providing an optical method for determining dissolved oxygen in fuel using the oxygen concentration dependence of the luminescence lifetime of an appropriate probe molecule doped in the fuel. The method of the invention is rapid, in that a single measurement can be made on a microsecond timescale and a signal comprising the average of 30 decays may be made in one second. The method is highly sensitive, non-intrusive, non-destructive, insensitive to thermal stressing of the fuel, less expensive than existing methods, and may be designed to obtain spatially-resolved profiling of oxygen concentration in fuel lines.
It is therefore a principal object of the invention to provide a method for determining dissolved oxygen in liquid fuel.
It is a further object of the invention to provide an optical method for the quantitative determination of dissolved oxygen in liquid fuel.
It is yet another object of the invention to provide a non-invasive method for determination of dissolved oxygen in liquid fuel.
It is a further object of the invention to provide a highly sensitive method for quantitatively determining the concentration of dissolved oxygen in liquid fuel by observing the laser excited fluorescence of a probe molecule in the fuel.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.